A method of authorizing a subscriber user equipment to access a Network Application Function, NAF, in a Generic Bootstrapping Architecture, GBA. The method includes, at a Bootstrapping Server Function, BSF, receiving a request for key material for the subscriber user equipment from the NAF, wherein the NAF is associated with one or more NAF identifiers, NAF_IDs, obtaining subscriber information from a Home Subscriber System, HSS, the information including one or more NAF_IDs that are valid for the subscriber, authorizing the subscriber user equipment on the basis of the identity of said NAF and the NAF_ID(s) included within the subscriber information. In the event that the subscriber user equipment is authorized, key material is derived using a NAF_ID that is contained within said subscriber information; and the key material is sent to the NAF.